A cathode ray tube generates an image-producing raster by means of a pair of deflection yokes which sweep the electron beam in vertical and horizontal coordinates respectively. Although electrostatic deflection is known, most yokes deflect the beam magnetically. Typically they do this by driving a sawtooth current through a deflection coil to generate a varying magnetic field which deflects the beam across the tube face.
In order to center the raster upon the CRT screen, such magnetic deflection systems may superimpose a steady magnetic field upon the varying field in order to move the center position of the beam to the geometric center of the screen. Several ways of doing this are known.
One involves the use of permanent magnets to impose a DC magnetic bias upon the neck of the CRT. Such systems involve cumbersome mechanical means for adjusting the position of the magnets.
Other approaches avoid these mechanical problems by electrically generating a steady-state magnet bias field. For example, a separate bias coil (not electrically connected to the deflection coil) may be mounted on the neck of the tube, and a constant current may be driven through this bias coil to generate a non-varying magnetic field. The magnitude of the bias field is then easily adjusted electrically, by adjusting the level and direction of the current in the separate coil. But the provision of a separate bias coil adds to the expense of the cathode ray tube.
There is another electrical approach to the problem of beam centering, one which does not require a separate bias coil. This involves driving a constant bias current through the deflection coil itself, the constant current being superimposed upon the varying deflection current which generates the raster sweep. The advantage of this approach is that it makes the deflection coil do double duty. But in this type of system it is necessary to isolate the DC bias circuit from the AC sweep circuit. Prior art proposals for accomplishing this have envisioned the use of a transformer to isolate the bias supply from the sweep current source. The transformer adds weight and expense, and also necessitates additional components for rectifying and filtering the AC voltage which is taken from the secondary of the transformer.
Accordingly, this invention contemplates an improved system for centering a magnetic CRT deflection circuit. The system is electrical, and therefore requires no permanent magnets or mechanical linkages. It uses the deflection coil itself for both sweep and bias, and so does not require a separate bias coil. It does not require a transformer for isolation, and thus also avoids the need for rectification and filtering of the bias supply.